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Session 31 - The Diffuse Interstellar Medium, Progress and Puzzles - I.
Oral session, Tuesday, June 11
Historical Society,
Contrary to the traditional picture of the interstellar medium in which ionized hydrogen is limited primarily to well defined Stromgren Spheres near the Galactic midplane and within a few tens of parsecs of O stars, most of the interstellar H II is in fact located far from the midplane and far from O stars, in an extensive, low density "intercloud" component that permeates the Galactic disk and lower halo. The existence of this warm ionized medium was first deduced 33 years ago by Hoyle and Ellis (Australian J. of Phys. 16, 1, 1963) from the spatial and spectral characteristics of Grote Reber's very low frequency (2-10 MHz) Galactic synchrotron data, and was later confirmed by pulsar dispersion measures and the detection of faint diffuse optical line emission from the interstellar medium. Within a few kpc of the sun this ionized gas is characterized by regions of nearly fully ionized hydrogen, having an electron density of 0.1 cm^-3, a temperature near 8000 K, and occupying 20 percent or more of the volume within a 2 kpc thick layer about the Galactic midplane. The mass of the H II is approximately 1/3 that of the H I, and the required power is equal to that from supernovae, making this gas a major component of the interstellar medium. Neither the source of the ionization nor its relationship to the neutral and the hot components have been established; however, the mere presence of this gas challenges our understanding of the composition and structure of the interstellar medium and the principal mechanisms of heating and ionization within it. Improved observational techniques for mapping and studying diffuse ionized gas, both in our Galaxy and in others, promise to provide us with new clues about the nature of large scale interstellar processes at work within the disks and halos of galaxies. This work has been supported in part by the National Science Foundation.
